Is there a way to create a C 'union'-like structure in Delphi? That
is, a structure that uses the same memory area?

The Delphi (Pascal/ObjectPascal) equivalent to a C-union structure is called a Variant
Record (not to be confused with the Variant "type" available in Delphi 2.0+). As
with a C-union, the Pascal variant record allows several structure types to be combined
into one, and all will occupy the same memory space. Look up the syntax declaration under
"Records" in the help file. But here's an example:

where the coordinates of the rectangle can be expressed using either four integer
values or two TPoints.

I realize this is pretty quick and dirty, so I suggest you refer to the help file for a
more in-depth explanation, or go to your nearest book store or library and look at any
Pascal book (not Delphi -- most won't explain this fairly esoteric structure). However, if
you're familiar with the C-union, this stuff should be an absolute breeze.